Method and apparatus for obtaining images from gas detectors by electron avalanches



June 10, 1969 A. LANSIART ETAL METHOD AND APPARATUS FOR OBTAINING IMAGES FROM GAS DETECTORS BY ELECTRON AVALANCHES Sheet Filed Nov. 21, 1966 June 1969 A. LANSIART ETAL 3,449,573

METHOD AND APPARATUS FOR OBTAINING IMAGES FROM GAS I DETECTORS BY ELECTRON AVALANCHES Filed Nov. 21, 1966 Sheet ,3 of 2 United States Patent US. Cl. 250-83.3 1 Claim ABSTRACT OF THE DISCLOSURE Images are obtained from gas detectors by the formation of electron avalanches. The voltage between the auxiliary electrode and the anode of the detector is adjusted to maximum for prevention of spark formation by 8 particles, X-ray photons or gamma-ray photons and the images of glow discharges caused by electron avalanches are recorded.

This invention relates to a novel method of utilization of a gas detector which permits visual detection of [3 particles as well as X-ray photons or 'y-ray photons and to a device for the practical application of said method.

Cross reference to related application A detector of known type which is described in allowed United States patent application Ser. No. 447,675, as filed by the present applicant on Apr. 13, 1965 under the title Particle Detector and as illustrated in FIG. 1 comprises a leak-tight chamber formed of a cylindrical casing 2, the top end of which is closed by a transparent glass plate 4 and the bottom end of which is constituted by a thin metallic cathode 6 such as a strip of aluminum foil for example. Said thin strip is supported by a plate 10 through which are formed parallel apertures and which performs the function of collimator. The plate referredto can be made either of lead or of stainless steel.

An auxiliary electrode in the form of a fine-mesh metallic grid 12 and a transparent anode 14 having a conducting surface (which may also be constituted by a fine-mesh grid or formed of conductive glass, for example) are placed within the gas-filled chamber above the cathode and parallel thereto.

Background of invention If the detector is filled with an inert gas, it is not possible to obtain the image of a radioactive source. On the other hand, if there is added a suitable quantity of organic vapor which absorbs the ultraviolet radiations of the gas, it is possible to produce discharges and even sparks in the presence of said source provided that the value of the auxiliary-electrode/anode voltage is judiciously determined.

Molecules of organic vapor are destroyed each time a particle is detected by the detector. But the production of sparks results in much greater destruction of these molecules and consequently in a limitation of the service which exists between the cathode and the grid. In the field just referred-to, the electrons initiate the process of cumulative ionization or electron avalanche which usually results in the formation of a spark and in the visualization of the particle.

When a spark is produced between the grid and the anode, the interelectrode capacitance which is a function of the electrode dimensions accordingly discharges; the value of resistance is so determined that the time constant of the recharging circuit is sufficiently high to make it unlikely that a further spark will again appear at the same point.

This recharging process consequently has a given time duration which corresponds to a relatively long dead time, thereby slowing down the triggering rate of the detector.

Summary of invention The present invention proposes a method of the type hereinbefore described for the utilization of gas detectors with a view to obtaining images caused by the formation of the electron avalanches aforesaid in such a manner as to circumvent the disadvantages which have been briefly outlined above and which arise from the formation of sparks.

The method in accordance with the invention for the utilization of a gas detector of known type for obtaining images resulting from the formation of electron avalanches consists in adjusting the voltage developed between the auxiliary electrode and the anode to the maximum permissible value for the prevention of spark formation under the action of 6 particles as Well as X-ray photons or 'y-ray photons, in amplifying the images of glow discharges caused by electron avalanches, then in recording the resultant images.

The invention is also directed to an assembly for the practical application of the above-mentioned method, characterized in that it comprises a gas detector comprising three electrodes, an image amplifier of known type which is adapted to transmit the images of glow discharges caused by electron avalanches and an apparatus for recording said images (such as, for example, a photographic camera).

The main advantages of this novel application lies in the fact that there is no discharge of the anode grid capacitance and that the dead time does not appear, with the result that there are no longer any disadvantages involved in increasing the pressure with a view to enhancing the quantum efiiciency of detection of particles.

In addition to these main arrangements, the invention is also concerned with a number of different secondary arrangements which are mentioned hereinafter and which relate to modes of application of the method according to the invention.

The technical characteristics of the invention will become more readily apparent from the following description of one exemplified embodiment which, as will be understood, it not intended to imply any limitation either in the modes of execution of the invention or in the potential uses thereof.

Brief description of the drawings Reference is had to the accompanying drawings, in which:

FIG. 1 is a sectional view of a gas detector as employed in accordance with the invention;

FIG. 2 is a general arrangement diagram of an assembly for the practical execution of the method according to the invention.

In order that the detector of FIG. 1 should operate a spark chamber, the filling-gas pressure and the value of the anode voltage which has to be stabilized are adjusted in such a manner as to obtain a suflicient number of 3 sparks per second in the presence of a radioactive source.

Under these conditions, an image caused by collected radioactive particles can be produced in a time interval of a few minutes. The sparks produced are in that case due to the breakdown of the dielectric strength of the gas. The quantity and distribution of the electrons which pass through the grid are sufficiently rich to ensure that a reduction of a few percent in the grid-anode voltage makes it possible to obtain only those sparks which are caused by primary tracks of ionizing particles. Said sparks can readily be observed and recorded by means of a photographic camera. The present applicant has sought to obtain similar results by making use of the same gas detector in such a manner as to obviate the grave disadvantages which arise from the dead time.

The applicant has found that, by reducing the amplitude of the grid-anode field to a slight extent in the presence of the same radioactive source, it is possible to avoid the formation of sparks. There nevertheless take place electron avalanches which result in emission of light. And while the quantity of light emitted is smaller than it would have been otherwise, it nevertheless remains larger than in the case in which the particles are transmitted directly to a scintillator of the thalliumactivated sodium iodide type, for example.

A study of the variations, as a function of the gridanode voltage, of the ratio p of the amplitude of the light signals obtained in studies of the same source by means of a gas detector which operates in the avalanche condition and by means of a scintillator of the type already defined makes it possible to draw up the following table:

Grid-anode voltage It has also been found as a result of these studies that there was no longer any dead time and that the increase in filling-gas pressure made it possible to increase the ratio p. It will be noted by way of example that, when the pressure is doubled, said ratio is multiplied by a factor which can vary between 1.5 and 2.

The utilization of a gas detector in the avalanche condition no longer permits of direct observation of the image or of direct recording by means of a camera, with the result that said image must undergo intermediate amplification.

FIG. 2 shows the complete assembly comprising a gas detector A which operates in the avalanche condition and an image amplifier which must be employed in order to obtain an accurate and visible image.

Similar elements which appear in both figures are designated by the same reference numerals. This assembly makes it possible to study a radioactive source 16 which is surmounted by a shield 18. These two elements are located above a gas detector of the same type as that which is shown in FIG. 1.

The cathode 6 and the stainless steel collimator 10 are brought to ground potential whilst the auxiliary electrode 12 and the anode 14 are respectively coupled to highvoltage and low-voltage sources HT and ET by means of resistors 20 and 22.

The image formed by the glow discharges caused by electron avalanches is transmitted to the camera 24 by the multi-stage secondary-emission image amplifier 26, the image observed is formed on the photocathode 28 of the image amplifier by means of an object-lens 30, and the image which is intended to be recorded by the camera appears on the screen 32 of the tube. A focusing solenoid 34 surrounds the amplifier tube 26.

The present applicant has designed a number of assemblies such as the unit which is shown in FIG. 2, in which the gaseous mixture employed was xenon and methylal, the partial pressures of which were respectively 730 torr and 30 torr for a total filling pressure of 760 torr. These units make it possible to obtain in approximately ten seconds images which exhibit strong contrasts and in which there can clearly be seen the useful components of images caused by the avalanche phenomenon of stray components which are due to amplifier noise.

What we claim is:

1. A method of utilization of a gas thru electrode detector of known type for obtaining images resulting from the formation of electron avalanches in an organic vapor and an inert gas mixture under pressure the steps of adjusting the voltage developed between auxiliary electrode and anode of the detector to the maximum permissible value for preventing the production of sparks under the action of ,8 particles and X-ray and y-ray photons, then amplifying the images of glow discharges caused by electron avalanche phenomena, and then recording the resultant lmages.

References Cited UNITED STATES PATENTS 3,337,733 8/1967 Charpak et al. 25083.6

ARCHIE R. BORCHELT, Primary Examiner. MORTON I. FROME, Assistant Examiner.

US. Cl. X.R. 

